1. Field of the Invention
This invention relates to a high pressure vapor discharge lamp which has a built-in igniter using a nonlinear capacitor.
2. Related Art
A conventional igniter for a high pressure vapor discharge lamp including a high voltage pulse generator using a glow lamp has problems such as poor operation stability and short lifetime. Therefore, an igniter using a nonlinear ceramic capacitor has come to be used. Such a nonlinear ceramic capacitor is mainly made of a ferroelectric substance such as barium titanate having nonlinear V-Q characteristics. In this igniter, a pulse voltage is generated every half cycle, by utilizing the saturation characteristics of the nonlinear capacitor and the inductance of a ballast or the like which is connected in series with the nonlinear capacitor. The pulse voltage thus generated is applied to a high pressure vapor discharge lamp, thereby starting it to operate. The construction of a high pressure vapor discharge lamp including such an igniter will be described with reference to FIGS. 1 and 2.
In FIG. 1, reference numeral 1 designates an arc tube for a high pressure sodium lamp, 2 designates a normally-closed thermally-activated bimetal switch, and 3 designates a nonlinear capacitor. The thermally-activated bimetal switch 2 and nonlinear capacitor 3 are connected in series to constitute an igniter which is connected in parallel with the arc tube 1. These components 1, 2 and 3 are housed in an outer bulb 4, thereby constituting a high pressure sodium lamp. Reference numeral 5 designates a ballast such as a choke coil, and 6 designates an AC power source.
Next, the operation of the thus configured high pressure sodium lamp will be described. When the power source 6 is turned on, the voltage of a positive half cycle is applied through the ballast 5 to the nonlinear capacitor 3, so that a charging current flows therethrough. The level of the charging current rapidly drops to zero when the nonlinear capacitor 3 is saturated with electric charges, or when the voltage reaches the saturation voltage of the nonlinear capacitor 3. At this time, the inductance of the ballast 5 causes a high positive pulse voltage to be generated. This pulse voltage and the voltage supplied from the power source are applied to the arc tube 1. Similarly, in the subsequent negative half cycle, a negative pulse voltage is generated. These pulse voltages cause the lamp to start to operate and light up. After the ignition of the lamp, the thermally-activated bimetal switch 2 receives heat from the arc tube 1 to open, thereby disconnecting the igniter from the main circuit.
The configuration of the high pressure sodium lamp having an igniter shown in FIG. 2 is the same as that of the lamp shown in FIG. 1, except that a bidirectional semiconductor diode switch 7 such as an SSS device is connected in series with the nonlinear capacitor 3 of the igniter. The semiconductor switch 7 is not located in the outer bulb 4, but located in a base 20 of the lamp.
The high pressure sodium lamp having the above-described configuration operates as follows: When the AC power source voltage in each cycle exceeds the breakover voltage of the semiconductor switch 7, the nonlinear capacitor 3 is rapidly charged, so that the voltage of the capacitor immediately reaches the saturation voltage, thereby rapidly interrupting the current. This results in the generation of a pulse voltage with a higher peak. Thus, this igniter is suitable for a high wattage lamp.
Such a high pressure vapor discharge lamp with a built-in igniter including a nonlinear capacitor, particularly a high pressure sodium lamp which requires a high ignition voltage, sometimes fails to light up. When such an ignition failure of the lamp once occurs, the built-in igniter continues to generate pulses. This continuous generation of pulses causes the following problems:
(1) Since the coil and core of the ballast are capacitively coupled to each other, the pulse energy generated through the inductance of the ballast leaks out to a metal housing of a lighting fixture in which the ballast is mounted. If the metal housing is not grounded, therefore, the leakage will give an electric shock to a human body.
(2) The pulse voltage is continuously applied to the base of the lamp or the metal part of the lamp holder. This is dangerous to a human body.
(3) The insulation of the ballast, wiring cables and a socket is deteriorated.
(4) A portion of the pulse energy is converted into high-frequency noise to be radiated outside, thereby causing the radio interference in television and radio receivers.